1. Field of the Invention
The invention relates to a method for determining the rotor resistance of a rotating field machine, in particular at stand-still or at low load, by means of a voltage model for the machine provided with current values and voltage values and a current model for the machine provided with current values and signals describing the rotor axis and a model parameter for the rotor resistance of the machine. The invention also relates to apparatus for carrying out the method.
2. Description of the Prior Art
For the field-oriented control or regulation of modern highly dynamic rotating-field machines, information regarding the position of the field axis is required. In order to avoid the installation of separate measuring probes in the machine, computing models are frequently used which compute the physical processes linked with the flux of the machine from readily accessible measurement variables.
Thus, it is possible, for instance, to determine from the machine voltage, the flux inducing this voltage. More particularly, the voltage yields, after subtracting the ohmic voltage drop due to a parameter r.sub.S for the stator resistance and the stray voltage drop defined by a parameter x.sub.st for the stray inductance, the EMF induced in rotor. The integral function of the EMF corresponds to the flux in the rotor of the machine. In such a "voltage model" the integration of the EMF can frequently be dispensed with since the latter represents merely a rotation by 90.degree.. At low speeds, however, the voltage model does not work satisfactorily.
In the so-called "current model", one starts out with the current fed into the machine and the position of the rotor axis, so that the flux induced in the rotor can be calculated, taking into consideration the rotor resistance. The rotor resistance itself, however, changes as a function of the operating temperature and is not accessible to direct measurement. This introduces an error into the values of the flux axis calculated in the current model.
In German Offenlegungsschrift 34 30 386 a method for operating a rotating field machine is described, in which the field angle is determined by means of the voltage model, and the voltage model itself is corrected by means of a current model. The rotor resistance parameter used in the current model is control. For this control series of square-ware pulses are added to the reference value for the magnetizing current. Therefore corresponding pulse-shaped changes of the magnetizing current occur in the machine which cause, by a smoothing brought about by the time constant of the rotor, a corresponding periodic change of the flux or the EMF, respectively.
The amplitude of the EMF vector or flux vector calculated in the current model changes with a time constant different from the time-dependent of the vector amplitude calculated in the voltage model, so that temporarily an amplitude difference occurs which depends on the error due to the rotor resistance. Thus, the amplitude difference correlated with the pulse-shaped supplemental reference values is determined and is compensated by readjustment of the rotor resistance parameter.
In this known method, a supplemental reference value with a harmonic waveform and a high frequency would lead to the situation wherein the flux in the machine changes only slightly and therefore, the error function which enters into the different dynamics of the two vector amplitudes, cannot be calculated with sufficient accuracy. Therefore, appreciable periodic changes of the flux are unavoidable which, while not influencing the active current, do influence the torque which is given by the product of the active current and the flux.